1. Field of the Invention
This invention relates to a spindle control system for machine tools, and more particularly to an inexpensive spindle control system which neither requires a spindle positioning pin or like mechanical parts nor calls for the provision of an additional detecting means on the spindle for stopping it at a specified rotational angular position.
2. Description of the Prior Art
In a machine tool, in particular, a machine tool provided with an automatic tool changer, after a series of machining operations of a work with a tool secured to the spindle, the tool is removed from the spindle and stored in a tool storage magazine, and then a different tool is selected and secured to the spindle for a subsequent machining operation. The tool stored in the tool storage magazine is selected again and secured to the spindle for machining as required. For maintaining high machining accuracies it is necessary that the tool be secured to the spindle always in the same angular relationship thereto.
Further, it is known that the spindle and the tool are keyed to each other so as to ensure transmission of the rotation of the former to the latter. To perform this, when the tool once withdrawn from the spindle is secured again thereto, they must be engaged with each other at a specified relative rotational angular position. Accordingly, in the case of stopping the spindle when required to change the tool, it is desired to stop the spindle at a specified rotational angular position.
In the prior art, for stopping the spindle at such a specified position, a switch is provided which is actuated when the spindle has reached the specified rotational angular position, and a lock pin is inserted into the spindle in response to the actuation of the switch to stop the rotation of the spindle and retain it at the specified position.
However, such a conventional mechanism requires the provision of a special switch on the spindle and a mechanical lock mechanism for holding the spindle at its stopped position, and hence has the defect that the apparatus is complicated and expensive.
Moreover, it is known to drive the spindle with a hydraulic spindle motor through transmission gears, as set forth in U.S. Pat. No. 3,704,510 by Robert K. Sedgwick et al. The spindle motor is controlled in speed so that the output voltage of a tachometer generator for detecting the actual speed of the spindle may become equal to a command voltage.
For stopping the spindle at a specified rotational angular position, a synchro (resolver) is coupled with the spindle and the spindle motor is stopped at a rotational angular position indicated by a reference voltage. That is to say, in the case of stopping the spindle at a constant position, an error operator for the speed control is disconnected from a servo amplifier of the spindle motor, and the reference voltage and the synchro and a discriminator are connected to the servo amplifier via contacts. The rotor of the synchro is driven by the spindle and the spindle motor is stopped at the position where the output from the discriminator becomes zero.
The above conventional mechanism requires a detector for the position control, that is, the snychro, in addition to a detector for the speed control, that is, the tachometer generator. The position control using the synchro involves many circuit elements and hence is complex. The reason is that there must be provided a circuit for generating a sine wave and a cosine wave for exciting the synchro, a circuit for detecting the phase of an output signal and so on. Additionally, adjustment of the synchro is troublesome and must be effected after checking the sine wave and the cosine wave through the use of an oscilloscope.